Medium Pressure Mercury Type H-2

The first medium pressure mercury lamps to be made in the USA were a direct copy of the original Osram 400-watt MA design, both physically and electrically. In America it quickly became known as the H-1. However it soon became apparent that a lower wattage lamp would be desirable, and work progressed independently on both sides of the Atlantic.

The European answer came in the form of a smaller 250W MA lamp, but the American developments resulted in the radically different H-2 lamp pictured here. The outer jacket of a medium pressure mercury lamp principally serves to minimise convected heat loss, and shield cold air draughts which influence mercury vapour pressure and light output. It adds some complexity to the lamp, and was temporarily sacrificed with the launch of this unusual lamp. Owing to the reduced mercury pressure in this design, it is not electrically compatible with any other 250W mercury product, however the diffuse arc stream does permit universal burning. Convected heat losses made it rather inefficient though, and within a few years it had been superseded by the 250W H-5 quartz arc tube lamp which reverted to using an outer bulb.

The single ended design of this H-2 arc tube is most unusual. All glass-metal seals are made at the cap end and a glass-sheathed tungsten wire, insulated from the arc, carries the current to the electrode at the far end. External black paint serves as a heat reflector to prevent mercury condensation in this region. Similar lamps were trialled by BTH Mazda in the UK and these employed conically-shaped arc tubes instead to combat the thermal problems of a bare arc tube, but due to troublesome heat loss issues they were never offered for sale.